1. Field of the Invention
This invention is related to an apparatus for transporting an electrically conductive material having the shape of a wafer by use of an electrostatic attractive force.
2. Description of the Related Art
There have been many types of carrying mechanisms employed for transporting a wafer, which is of relatively light weight, such as a semiconductor wafer. Some of the conventional transporters are shown in FIG. 1 through FIG. 5.
FIG. 1 shows a belt conveyer system where a wafer to be carried is placed on a belt which is driven by a motor and a drive mechanism.
FIG. 2 and 3 show a walking beam system where the beam makes a square motion like a fork lift, i.e., the beam moves in the direction of the arrow "a" to lift the wafer 6 sitting on a stage, then transports the wafer horizontally in the direction of the arrow "b", goes down in the direction of the arrow "c" and leaves the wafer on th stage and returns in the direction of the arrow "d" to the starting position.
FIG. 4 shows an air bearing system where an air jet is blown from a plurality of air nozzles provided in the stage by which the wafer is to be transported. The direction angle of the air jet is slanted so that the wafer floats as well as being driven toward the destination.
FIG. 5 shows a linear motor system where the wafer or a tray carrying the wafer thereon is magnetically driven by a linear motor.
Another system is a so-called electrostatic chuck, which is typically disclosed by Abe in unexamined patent publication Tokukai No. Sho 57-64950 in Japan and is composed of plural flat electrodes covered with a dielectric layer, with a high voltage applied between the electrodes. An electrostatic attractive force induced between the wafer and the electrodes by the applied voltage holds the wafer.
Problems of these conventional systems are as follows: In the systems of FIG. 1 through 3 and the electrostatic chuck, the surface of the wafer must contact a foreign material, such as the belt or the beam, etc., and the system becomes bulky. In FIG. 4 the edges of the electrodes must contact side walls (which are not shown in FIG. 4), by which the wafer is guided, i.e., is prevented from deviating from its direction. In FIG. 5 the wafer or a tray must be of a magnetic or electrically conductive material and the mechanism becomes complicated, because the semiconductor wafer, now in a major use, is not magnetic, nor sufficiently conductive. Furthermore, there are problems in that the mechanically moving parts are apt to cause dust or an impurity material which are harmful in semiconductor production. Machining of the slanted air bearing nozzles requires very sophisticated technique causing an increase of the cost of the production facilities. Furthermore, either of these conventional systems consumes a considerable amount of electric power.